The magnetic pickup has become the standard pickup for the guitar as well as for other stringed instruments. Recently however, there has been some effort to overcome its inherent disadvantages which include a weak signal, sensitivity to stray fluctuating magnetic fields, limited frequency response, and dependency on metal strings.
Optoelectronic pickups have been devised in an attempt to overcome some of these problems. They generally consist of a light emitter-detector combination configured in such a way as to detect the motion of a string as it moves through, and interferes with, the light path between the emitter and the detector.
Although they may generally overcome some of the problems of the magnetic pickup, they exhibit certain inadequacies and even generate problems of their own. For example, none of these new pickups guarantee a linear relationship, or any definitive relationship for that matter, between the actual position or speed of the string and the electrical output signal of the pickup. They also tend to combine the two dimensions of motion of a cross section of a vibrating string into a one dimensional output signal in a vague and often undesirable way.
No well defined continuous relationship between string motion and signal output exists in these systems. For example, many such pickups work simply by relying on the string to chop a beam of light, indicating that the relationship between the string motion and position and output signal is highly nonlinear and noncontinuous.
Intentionally or unintentionally combining the electrical representation of the two dimensions of the string motion in the output signals of these pickups can produce undesirable effects. When they are combined, the portion of the output signal due to string motion along one axis of motion can interfere with the portion of the output signal due to string motion along another axis causing harmonic distortion of the output signal. This is an important consequence since it is expected to have the string moving significantly in two dimensions whenever its vibrating. The belief that the string only vibrates significantly along one axis after it is strummed or plucked is really a misconception. In fact it is actually quite difficult to intentionally make it vibrate in this manner. During normal guitar playing the strings are typically going through a variety of complex and changing paths including elliptical, circular, etc.
A nonlinear relationship between the string position and the output signal also generates harmonic distortion. This is a principal reason that a pickup should have an output signal that is a definitive linear representation of the relative position of the string being observed.
U.S. Pat. No. 3,733,953 issued May 22, 1973 to Ferber discloses an optoelectronic pickup that operates on the principal of a string intersecting the path of a light beam from a light emitting device toward a light-detecting device.
U.S. Pat. No. 4,563,931 issued Jan. 14, 1986 to Siebeneiker discloses a system for scanning the vibrations of a mass. It includes a source of scanning radiation, and the umbra behind the mass and the surrounding illuminated area is collected by a focusing lens and projected onto an optoelectronic converter.
In the ensuing description of the present invention, the axis of a string or the longitudinal axis of a string shall be understood to mean the imaginary line through the center of the stationary string when mounted taught. When referring to a cross section of a string it will be meant the intersection between the string and a two dimensional plane to which the axis of the string is normal. To define a relative string position or a relative position of a string, first consider the center point of a given cross section of a mounted string. Now consider the position of this point relative to a given second point that is also on the same cross sectional plane that contains the string cross section. Assume also that this second point is fixed within the reference frame of the string's mount. The position of the first point relative to the given second point shall be called the relative string position. Typically then, whenever discussing a relative string position it will be understood to be with respect to a given cross sectional plane, and more than likely to be where an optical pickup is to be placed.
As a result of the inability of current optoelectronic pickup systems to isolate one or more orthogonal components of string position or motion, they are susceptible to harmonic distortion and other undesirable effects. This is one of the main reasons it is important to be able to break up this relative position or motion into components along one or two orthogonal axes and then translate these components into one or two respective output signals.
Once these components are separated, it is also important to provide a linear, or proportional relationship between the relative string position and the output signal.
The prior art does not address the problem of the effects of interference between the two orthogonal components of relative string position or motion. Nor does it sufficiently teach how to construct pickups with definitive linear relationships between the string position and the electrical output signal.
Still other optoelectronic pickup systems are more concerned with providing piecewise information about string motion rather than generating a signal analogous to a more complete description of the trajectory of a vibrating string. This may be acceptable for what they were intended for, which typically is generating only that information about string plucking needed by synthesizers. For instance, U.S. Pat. No. 4,730,530 issued Mar. 15, 1988 to Bonanno discloses a synthesizer guitar controller pickup method for generating control signals for a synthesizer.
Although there is sometimes a narrow linear range of operation for existing optoelectronic pickups for guitar synthesizers, for the majority of typical string positions they are providing only a simple control signal to a synthesizer and so could not be used to replace a standard magnetic pickup for nonsynthesizer type of playing.
Another problem common to all previous optoelectronic pickups is susceptibility to stray light. The typical solution to this so far has been to enclose the pickup system in some sort of light-tight shield. This adds to the cost and complexity of the system however, and since the string must pass through this enclosure on at least one side, there must always be an opening where stray light could enter. Stray light can cause problems in at least two ways. First, it can add to an undesirable DC bias at the detector output which will change for different room light levels. Second, any ambient light with AC components in the audio range e.g. florescent lights, lights with dimmers, infrared remote controls or just lights being turned on or off could be audible at the final output.
Present optoelectronic pickups still at some point have to convert the optical signal to an electronic signal for use by external equipment. The leads that carry this output signal are no more immune to electrical interference than pickups that use nonoptical means. It is desirable to significantly reduce or eliminate this noise.
Also, this type of pickup requires a source of electrical power for the light source. Therefore there may be a need for an efficient way to use this power since it may be desirable to power the pickup with a battery pack instead of running an extra set of power leads off of the guitar.